/******************************************************************************
* FILE: 12-cond.c
* DESCRIPTION:
*   Example code for using Pthreads condition variables.  The main thread
*   creates three threads.  Two of those threads increment a "count" variable,
*   while the third thread watches the value of "count".  When "count" 
*   reaches a predefined limit, the waiting thread is signaled by one of the
*   incrementing threads. The waiting thread "awakens" and then modifies
*   count. The program continues until the incrementing threads reach
*   TCOUNT. The main program prints the final value of count.
* SOURCE: Adapted from example code in "Pthreads Programming", B. Nichols
*   et al. O'Reilly and Associates. 
* LAST REVISED: 10/14/10  Blaise Barney
******************************************************************************
  pthread.h should be first otherwise compile with -D_THREAD_SAFE 
*/
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

#define NUM_THREADS  3
#define TCOUNT 10
#define COUNT_LIMIT 12

/* shared structure */
int count = 0;
pthread_mutex_t count_mutex;
pthread_cond_t count_threshold_cv;

void *inc_count(void *t) {
   int i;
   long my_id = (long)t;
   
   for (i=0 ; i < TCOUNT ; i++) {
      pthread_mutex_lock(&count_mutex);
      count++;

      /* 
	 Check the value of count and signal waiting thread when condition is
	 reached.  Note that this occurs while mutex is locked. 
      */
      if (count == COUNT_LIMIT) {
	 printf("inc_count(): thread %ld, count = %d  Threshold reached. ",
		my_id, count);

	 pthread_cond_signal(&count_threshold_cv);

	 printf("Just sent signal.\n");
      }
      printf("inc_count(): thread %ld, count = %d, unlocking mutex\n", 
	     my_id, count);
      pthread_mutex_unlock(&count_mutex);

      /* Do some work so threads can alternate on mutex lock */
      sleep(1);
   }
   pthread_exit(NULL);
}

void *watch_count(void *t) {
   long my_id = (long)t;

   printf("Starting watch_count(): thread %ld\n", my_id);

   /*
     Lock mutex and wait for signal.  Note that the pthread_cond_wait routine
     will automatically and atomically unlock mutex while it waits. 
     Also, note that if COUNT_LIMIT is reached before this routine is run by
     the waiting thread, the loop will be skipped to prevent pthread_cond_wait
     from never returning.
   */
   pthread_mutex_lock(&count_mutex);
   while (count < COUNT_LIMIT) {
      printf("watch_count(): thread %ld Count= %d. Going into wait...\n", 
	     my_id,count);

      pthread_cond_wait(&count_threshold_cv, &count_mutex);

      printf("watch_count(): thread %ld Cond signal received. Count= %d\n", 
	     my_id,count);
      printf("watch_count(): thread %ld Updating the value of count...\n", 
	     my_id,count);

      count += 125;

      printf("watch_count(): thread %ld count now = %d.\n", my_id, count);
   }
   printf("watch_count(): thread %ld Unlocking mutex.\n", my_id);
   pthread_mutex_unlock(&count_mutex);
   pthread_exit(NULL);
}

int main(int argc, char *argv[]) {
   int i, rc; 
   long t1=1, t2=2, t3=3;
   pthread_t threads[3];
   pthread_attr_t attr;

   /* Initialize mutex and condition variable objects */
   pthread_mutex_init(&count_mutex, NULL);
   pthread_cond_init (&count_threshold_cv, NULL);

   /* For portability, explicitly create threads in a joinable state */
   pthread_attr_init(&attr);
   pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
   pthread_create(&threads[0], &attr, watch_count, (void*)t1);
   pthread_create(&threads[1], &attr, inc_count, (void*)t2);
   pthread_create(&threads[2], &attr, inc_count, (void*)t3);

   /* Wait for all threads to complete */
   for (i = 0 ; i < NUM_THREADS ; i++) pthread_join(threads[i], NULL);

   printf ("Main(): Waited, joined with %d threads. Final count = %d. Done.\n",
	   NUM_THREADS, count);

   /* Clean up and exit */
   pthread_attr_destroy(&attr);
   pthread_mutex_destroy(&count_mutex);
   pthread_cond_destroy(&count_threshold_cv);
   pthread_exit (NULL);
}
